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A
research team at JILA has developed a new method for making
and analyzing an unusual floppy molecule. Shown above
with their experimental apparatus are (from left) Chandra
Savage, Erin Whitney, Feng Dong, and David Nesbitt.
Photo
by Jeff Fal, University of Colorado, Boulder
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a high resolution version of this image.
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Boulder,
Colo.–A laboratory method developed for making and analyzing
cold, concentrated samples of a mysterious “floppy”
molecule thought to be abundant only in outer space has revealed
new data that help explain the molecule’s properties.
The advance,
described in the Jan. 6 issue of Science,* is a step
toward overcoming a decades-old challenge in chemistry—explaining
reactions that occur within very cold clouds among the stars,
and perhaps for developing new chemical processes. The paper
combines experiments performed by David Nesbitt and colleagues
at JILA, a joint institute of the Commerce Department’s
National Institute of Standards and Technology (NIST) and
University of Colorado at Boulder, with theoretical predictions
made with Joel Bowman at Emory University in Atlanta, Ga.,
and Anne McCoy at The Ohio State University in Columbus, Ohio.
Most
molecules have a rigid three-dimensional (3D) structure. The
subject of the new study is “protonated” methane,
which contains one carbon atom and five hydrogen atoms, one
of which is ionized, leaving nothing but a proton (a particle
with a positive charge). The five protons from the hydrogen
atoms scramble for four bonds around the molecule as if playing
a continuous game of musical chairs. In the process, the molecule
classically vibrates and rotates in a bizarre manner, morphing
between several 3D structures with nearly identical energy
levels. (Animation available at http://www.nist.gov/public_affairs/images/floppy_animation.htm. Requires the free Quicktime player).
Chemists have spent decades trying to explain why and how
this occurs, a challenge that has seemed insurmountable until
recently.
Protonated
methane is a so-called “super acid.” This class
of molecule has been shown to be more than a million times
more powerful than conventional acids and is more effective
in inducing reactions that produce solvents and many other
important industrial products.
Many theories have been published on the puzzling behavior
of this charged molecule (or ion), but experiments must be
done to match the ion’s energy characteristics with
its physical motions, and such data are difficult to collect
and understand. In particular, scientists are interested in
how the molecule absorbs different wavelengths of infrared
(IR) light, which provides clues about nuclear motion and
chemical bonds and structures.
The JILA method
generates concentrated amounts of the ion at cold enough temperatures
to simplify the complex IR spectrum so it can be analyzed.
The data strike a balance between detail and simplicity, providing
useful information that is still challenging but easier to
understand than ever before. This enabled the authors of the
Science paper to match predicted changes in energy to specific
vibrations and partially characterize the ion's structure
and dynamics. For example, they were able to correlate one
intense spectral feature to a transition between two 3D structures
with equivalent energy levels.
Previously published
spectra of this molecule have either been too low resolution
to “see” this motion, or too hot (and therefore
too complex) to analyze.
“The experiments
have provided the first jet-cooled, high-resolution spectrum
of this highly fluxional molecule,” says Nesbitt, a
NIST Fellow who led the JILA experimental team. “This
has been among the most sought-after IR spectra since the
first appearance of this molecule in mass spectrometers over
50 years ago. This is a problem that has occupied many careers;
every piece helps.”
The JILA method
involves making methane gas at high temperature and pressure,
and expanding it into a vacuum to cool the molecules to 10
K (-442 degrees F). The cold molecules then file through an
opening just 1 millimeter wide, where they are hit with a
“lightning bolt” of electrical current that generates
high concentrations of highly reactive ions. The key to mass
production is to surround the molecules with enough electrons
to make the entire gas mixture neutral in charge, Nesbitt
says.
For the analysis
step, JILA scientists shine an infrared laser on the cold
ions, and detect the light that passes through. The light
that is lost, or the small amount absorbed by the molecules,
is analyzed to obtain a pattern of absorption at different
wavelengths. The technique is very sensitive, thanks to methods
for detecting trace absorption of the laser light and manipulating
the electrical discharge to maximize the ion concentration
levels.
Future and ongoing
studies will focus on matching the ion’s IR absorption
characteristics with its rotational structure, including end-over-end
tumbling. “Protonated methane still has a few tricks
up its sleeve,” Nesbitt cautions.
The research
was supported in part by the National Science Foundation,
Office of Naval Research, and Air Force Office of Scientific
Research.
As a
non-regulatory agency of the Commerce Department’s Technology
Administration, NIST promotes U.S. innovation and industrial
competitiveness by advancing measurement science, standards
and technology in ways that enhance economic security and
improve our quality of life.
*X. Huang,
A.B. McCoy, J.M. Bowman, L.M. Johnson, C. Savage, F. Dong,
and D.J. Nesbitt. 2005. “Quantum deconstruction of the
infrared spectrum of CH5+”. Science. Jan. 6.
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